Lime has long been known to be useful in various structural applications. Typically, the starting substance for lime applications is either hydrated (slaked) lime or unhydrated (quicklime). Several patents disclose methods and equipment for hydrating lime, including U.S. Pat. Nos. 783,570, 1,579,766, 1,664,598, and 2,044,553.
In addition to lime, fly ash has, to a limited extent, also been used to stabilize road or rail road beds. Such, lime-fly ash mixes have been used in the dry form and, to a limited extent, they have also been used in a high pressure injection slurry medium.
Typically, however, such dry mixes as well as the slurries require special care in mixing and handling. For example, systems that provide for the distribution of powdered constituents on the earth, followed by the addition of water and subsequent mixing into the earth create substantial dusting problems which are exacerbated by the caustic nature of the dust. In addition, when quicklime is used, such systems may pose short-term hazards to operators, as the hydration reaction is exothermic. Finally, the high pressure injection slurries are not directed to the surface layers of the soil. They also present problems of a fast setting time that requires immediate injection. Some examples of patents disclosing various lime/ash mixtures are as follows:
U.S. Pat. No. 4,496,267 discloses prereacting a mixture of incinerator ash, rather than fly ash, and lime for several days, and then adding time and water to the prereacted mixture and working it into the surface of the road bed.
U.S. Pat. No. 4,105,463 discloses a composition comprising about 2-8 percent time, about 8-36 percent fly ash, from 66 to 90 percent aggregate, and 0.5-4 percent sludge. These are mixed to produce a composite to which water can be added to form a cementitious product.
U.S. Pat. No. 3,076,717 discloses a composition having little early compressive strength but having very high early bearing power. The composition comprises 10-30 percent of a pozzolan such as fly ash, 70-90 percent of an aggregate having a specific particle size distribution, about 2-6 percent lime, and excess water of about 7-10 percent.
U.S. Pat. Nos. 2,564,690, 2,698,252, and 2,815,294 disclose compositions comprising lime, fly ash and fine aggregate for stabilizing soil having a particular fineness.
U.S. Pat. No. 4,329,090 discloses a method of mixing quicklime and water to form a hot hydrated lime slurry and working the hot slurry into the earth before it cools. The patent also discloses parts of an apparatus for accomplishing the mixing of quicklime into the water in a particular manner. Specifically, the '090 patent teaches injecting the quicklime below the surface of the water and stirring the mixture to prevent any localized concentration of quicklime particles, the hydration of which might be undesirably violent. Likewise, systems exist which provide for the injection of a lime-fly ash slurry into the subsurface layers of the earth. These systems not only require special injection equipment for piercing the upper layers of the earth, but, typically, they also require that the slurry be mixed only minutes before it is injected in order to avoid undesirable setting of the mixture before injection. This greatly reduces the flexibility of the operation with respect to supply, transport and procedure.
U.S. Pat. No. 4,084,381 discloses a lime-fly ash slurry comprising about 20-60 percent solids. The slurry is injected at predetermined, spaced-apart locations, at predetermined depths up to 10-20 feet. Of the solids, the lime comprises about 25-50 percent by weight. The fly ash is preferred to be the self-hardening type, having preferably greater than 10 percent by weight calcium oxide.
U.S. Pat. No. 4,233,015 discloses a method of treating subsurface layers of the earth by mixing quicklime and water to form a hot hydrated lime slurry and injecting the hot slurry into the earth before the slurry cools. Specifically, the '015 patent teaches injecting the quicklime below the surface of the water and using a series of rotating paddles to stir the mixture and to prevent any localized concentration of quicklime particles.
U.S. Pat. No. 4,871,283 discloses injecting a mixture consisting essentially of water, particulate hydrated lime and particulate fly ash, wherein the solids are present as about 25 to 200 percent by weight of the water. The disclosed ratio of lime to fly ash ranges from 3: 1 to about 1: 10. It is preferred that Type C fly ash having at least 5 percent and preferably at least 10 percent calcium oxide be used, and it is stressed at column 7, lines 16-22, that the injection should occur within ten minutes of mixing of the components.
U.S. Pat. 4,871,283 also specifically teaches that Type F fly ash cannot be used in the patented method without other additives. This is directly contrary to the nature of the present invention.
Many states have prior art specifications relating to the use of lime and of fly ash as stabilizing systems for road beds. For example, the Texas Department of Transportation has issued "Standard Specifications for Construction of Highways, Streets and Bridges. Its 1993 edition, at page 228, continues to call for the application of fly ash in "dry form" only. As will be demonstrated, the present invention will reduce both the cost and the environmental hazards that result from such specifications.
The present invention avoids the problems associated with the prior systems and provides a safe, economical method for stabilizing a road bed. Other objects and advantages of the invention will appear from the following description.